1. Field of the Invention
The present invention is directed to cold rolled steel sheet products and a method for making the same. In particular, the cold rolled steel sheet has excellent formability, bake hardenability, and dent resistance, and is resistant to aging.
2. Description of Related Art
Currently, high strength steel sheets, especially IF (interstitial free) rephosphorized Al-killed steel sheets with high yield and tensile strength, are being used by automobile manufactures to reduce vehicle weight and improve the mileage economy. Due to their relatively high yield strength, the steel sheets do not conform closely to stamping dies during stamping or forming, and thus sometimes cause undesirable surface deflection in the formed parts. Therefore, steel sheets with low yield strength before stamping and high yield strength in the final finished products are desired.
For this reason, steel sheets having bake hardenability and superior press shapability have been developed in the recent years. As used herein, the term bake hardenability refers to the hardening or strengthening that occurs during a paint baking or coating treatment, in which the steel sheet is typically held for about 20 to 40 minutes at a temperature ranging from 250 to 450.degree. F. (121 to 232.degree. C.). Due to such bake hardening or strengthening, this type of steel sheet can provide desired excellent dent resistance in the final product. A key characteristic of bake hardenable steel sheet to be used in producing automobile panels is that it should have high ductility and thus excellent formability prior to the bake hardening being conducted. This prolongs the life of forming tools and enables various different types of shapes to be easily produced, in that the press forming or shaping is conducted prior to the bake hardening step.
The previous research and development in this field has resulted in several methods for producing cold-rolled bake-hardenable steel sheets using a continuous, or in-line, annealing approach, several examples of which are discussed below.
U.S. Pat. No. 5,656,102 and U.S. Pat. No. 5,556,485 to Taylor et al. disclose effective amounts of vanadium in low carbon steels to produce an improved bake hardenable product for automotive use. The use of vanadium in the alloy steel chemistry controls bake hardenability, permits solution annealing at lower temperatures in its manufacturing sequence, and enables the use of a composition range which is more easily cast within desired limits and causes less variation in final mechanical properties. The effective annealing temperature range for this steel can be as low as around 1450.degree. F. (788.degree. C.) and up to about 1650.degree. F. (899.degree. C.). The solution annealing temperature is preferably within the range of 1500 to 1550.degree. F. (816 to 843.degree. C.), according to these patents.
U.S. Pat. No. 5,486,241 to Ushioda et al. discloses a non-aging extremely low carbon (0.0001 to 0.0015% C) ferritic single-phase cold-rolled steel sheet or hot dip galvanized steel sheet for deep drawing, and which has fabrication embrittlement resistance and paint bake hardenability. A continuous annealing process is conducted after the cold rolling of the steel sheet.
U.S. Pat. No. 5,470,403, European Patent 0,620,288 A1, and European Patent 0,608,430 A1 to Yoshinaga et al. disclose an extremely low carbon cold-rolled steel sheet and a hot dip zinc-coated cold-rolled steel sheet with bake hardenability characteristics. For this product, at least one element selected from the group consisting of Ti and Nb is used in the alloy chemistry. A continuous annealing procedure is also employed, with an annealing temperature from at least the a.fwdarw.y transformation point to up to the A.sub.c3 transformation point.
U.S. Pat. No. 5,356,494 to Okada et al. discloses a high strength cold-rolled steel sheet having non-aging properties, drawability, and bake hardenability characteristics. This steel sheet has a dual-phase structure, and is produced by preparing a hot-rolled steel sheet, cold rolling the hot-rolled steel sheet at a rolling reduction not smaller than 60%, continuously annealing the cold-rolled steel sheet at a temperature which is not lower than the a.fwdarw.y transformation start temperature, but which is also below the A.sub.c3 transformation temperature, and cooling the continuously annealed steel sheet at a rate not less than 9.degree. F./sec (5.degree. C./sec), but not greater than 180.degree. F./sec (100.degree. C./sec).
U.S. Pat. No. 5,123,969 to Chou discloses a cold-rolled steel sheet which has good bake hardenability, good dent resistance, and a low yield ratio. After melting, continuous casting, hot rolling, coiling and cold rolling, the steel sheet is soaked at a temperature ranging from 1436.degree. F. (780.degree. C.) to 1652.degree. F. (900.degree. C.) for less than five minutes preceding an intercritical (a+y) continuous annealing treatment.
U.S. Pat. No. 4,750,952 to Sato et al. discloses a cold-rolled steel sheet for deep drawing having improved bake hardenability. Titanium is added to this steel, and restricted to a specific range in consideration of the sulfur and nitrogen amounts. Such a cold-rolled steel sheet is obtained by continuously annealing the steel sheet after the cold rolling, provided that a residence time over a temperature region above recrystallization temperature is within 300 seconds.
U.S. Pat. No. 4,859,931 to Yasuda et al. provides a method for producing a thin bake hardenable cold-rolled steel sheet. This patent discloses an effective compounding amount of Ti which acts to fix the C, S and N contained in the steel, and a continuous annealing condition properly selected based upon the effective amount of Ti.
U.S. Pat. No. 4,496,400 to Irie et al. relates to a thin cold-rolled steel sheet suitable for external automotive plate. This patent discloses an effective compounding amount of Nb, which acts to fix C and N in the steel in the presence of a proper amount of Al, and a continuous annealing condition which produces the desired results with the addition of Nb.
U.S. Pat. No. 4,410,372. to Takahashi et al. discloses a process for producing deep-drawing, non-aging, cold rolled steel strip having paint bake hardenability, by continuous annealing the steel strip. In this patent, the cold-rolled steels are limited to Al-killed steels containing 0.001-0.01% C, not more than 1.5% Mn, 0.005-0.20% Al, not more than 0.007% N, and B in amounts determined by the ratio of B/N ranging from 0.5 to 2.5, and optionally containing not less than 1% Si and 0.04 to 0.12% P.
U.S. Pat. No. 4,050,959 to Nakaoka et al. provides a process of making a high strength cold reduced steel sheet having bake hardenability and non-aging properties. In this patent, the chemical composition is substantially controlled such that Mn is in the range of 10.times.[S]% up to 2.99%, N is in the range 0.003 to 0.02% and Al is less than 5.times.10.sup.-4 /[N]%. The cold reduced steel is subjected to a full continuous annealing process comprising a heating step to heat the strip to a temperature between A.sub.c1 and 1652.degree. F. (900.degree. C.) within 5 to 180 sec., a rapid cooling step from the heated temperature to substantially room temperature by water-spray, a reheating step to heat the strip from room temperature to a temperature in the range of 302.degree. F. (150.degree. C.) to 842.degree. F. (450.degree. C.) within 5 to 300 sec., and then a final cooling step.
U.S. Pat. No. 3,904,446 to Uchida et al. discloses a process of making high strength cold-rolled steel having bake-hardening characteristics. In this patent, the chemical composition is substantially controlled such that carbon is in the range of 0.04 to 0.12% C and manganese is in the range of 0.1 to 1.60%. After being cold reduced, the steel strip is continuously heated to a temperature in the range of 1292 to 1652.degree. F. (700 to 900.degree. C.), is then rapidly cooled by a jet of water, and is then reheated to a temperature in the range of 356 to 752.degree. F. (180 to 400.degree. C.) and held for 2 to 300 seconds at that temperature to leave a portion of the carbon in solution in the steel.
U.K. Patent GB 2,234,985 to Okamoto et al. relates to a bake hardenable steel of a composition, by weight, of 0.0010-0.0030%C, 0.04-0.30% Mn, 0.04-0.20% P, 0.003-0.015% Si, at most 0.15% soluble Al, at most 0.0020% N, and 0.003-0.025% Ti, and requires a specific relationship between N, Ti and S. The steel may also contain Nb and/or V, and optionally B, the balance being Fe and unavoidable impurities.
UK Patent GB 2,101,156 to Shibata et al. discloses a process for producing deep-drawing, non-aging cold-rolled steel strip having bake hardening properties. The process subjects the starting material to ordinary hot and cold rolling operations, and then the strip is soaked at a temperature in the range of from 1346.degree. F. (730.degree. C.) to the A.sub.3 point by a continuous annealing process. The strip is rapidly cooled from a temperature between the soaking temperature and 842.degree. F. (450.degree. C.) down to a temperature not higher than 482.degree. F. (250.degree. C.) with an average cooling rate of not less than 108.degree. F./sec (60.degree. C./sec).
All of the above patents or publications are related to the manufacture of cold-rolled bake hardenable steel sheets using a continuous annealing method. Compared to batch annealing, continuous annealing can provide steel sheets which exhibit more uniform mechanical properties, better flatness and cleaner surface. Flowever, the drawability and anti-aging properties of these steel sheets are inferior to those produced by batch annealing, due to the rapid heating and cooling cycles encountered in continuous annealing. Not enough solute carbon and nitrogen can be fixed as carbides, nitrides or carbonitrides by making the solute carbon and nitrogen precipitated during cooling step of continuous annealing. As a result, a large amount of solute carbon and nitrogen remains in the annealed steel sheet. Therefore, when the annealed steel sheet is left to stand for a long period of time before the steel sheet is pressed, the steel sheet ages at room temperature.
As also indicated in these patents or publications, very tight chemistries and processing controls are necessary for the production of bake hardenable steel sheets using a continuous annealing approach. In order to improve formability when continuous annealing is to be employed, ultra low carbon and nitrogen concentrations are needed, which, in turn, requires advanced steelmaking equipment and increases the production cost. Furthermore, for the purpose of controlling the stability of carbon and nitrogen, and thus the bake hardenability and anti-aging properties of the steel sheet, certain amounts of expensive alloys, such as titanium, niobium and vanadium, are usually added to the steel. This further increases the manufacturing cost.
U.S. Pat. No. 4,339,284 to Hashimoto et al. relates to a process which employs batch annealing technology. This patent discloses a method of producing non-aging cold rolled steel sheets capable of being deep drawn, wherein an extra low-carbon steel is melted together with niobium. The molten steel is made into an ingot, the ingot is slabbed, and then the slab is subjected to a hot rolling, a cold rolling and a batch annealing according to a common method. The patent is not concerned with bake hardenability of the steel sheet.
U.S. Pat. No. 4,313,770 to Takahashi et al. also involves batch annealing. The method disclosed in this patent comprises hot rolling, pickling, cold rolling, then passing the resulting steel strip to a batch annealing furnace in which the steel strip is subjected to recrystallization annealing by heating it at a temperature lower than 1400.degree. F. (760.degree. C.), but higher than the recrystallization temperature of the steel, and cooling it in the temperature range from 932 to 392.degree. F. (500 to 200.degree. C.) at an average cooling rate of 18 to 450.degree. F./hour (10 to 250.degree. C./hour), and then temper rolling the annealed steel strip.
As noted in this patent, the total elongation of the steel sheets produced using the above annealing cycle is often below 35%. This has been borne out in testing conducted in conjunction with the development of the present invention, wherein similar and even lower elongation values were obtained. The steel sheets obtained by the method in that patent also can exhibit aging at room temperature.
Despite the concerted activities in obtaining deep drawing, bake hardenable steel sheet, as evidenced in part by the relatively large number of patents noted above, a need still exists to develop new methods which improve the formability and non-aging property of cold-rolled steel sheet, in order to meet current shaping requirements for automobile, electrical appliance and building components.
The present invention has as a principal object thereof the provision of a batch annealing method, which has less demanding chemistry and processing requirements, for producing cold-rolled steel sheet and zinc or zinc-alloy coated cold-rolled steel sheet having improved formability, and excellent bake hardenability, dent resistance and non-aging properties.
A further object of the present invention is to provide a practical manufacturing method for making cold-rolled steel sheet and zinc or zinc-alloy coated cold-rolled steel sheet having improved formability and excellent aging resistance prior to forming. This method has less demanding processing requirements and can be carried out using less expensive steelmaking equipment.
Another object of the present invention is to provide a cold-rolled carbon steel sheet and coated cold-rolled carbon steel sheet having excellent bake hardenability, dent resistance, press shapability and non-aging property. This type of steel sheet is less expensive due to the less demanding chemistry requirements, neither requiring extra low carbon level nor containing expensive alloy elements, such as Ti, Nb and V.
Other objects and advantages of the present invention will become apparent from the description that follows.